This invention relates to manufacturing printed wiring boards and electrical components.
Printed wiring boards (PWB) perform several indispensable functions in electronic devices of all kinds. First, individual electrical components, e.g., specially packaged integrated circuits, resistors, etc., are mounted or carried on the surface of the flat, usually sturdy, card-like board. Thus, the PWB serves as a unitary mechanical support for the components. Secondly, using chemically etched or plated conductor patterns on the board's surface, the PWB forms the desired electrical connections between the components. Furthermore, the PWB often includes metal areas serving as heat sinks for high power or thermally sensitive components.
As the use of integrated circuits has grown, the higher density of connections between components has necessitated double-sided PWB's in which additional interconnections are made employing conductor patterns on the other side of the board. This trend has been extended to boards having many layers of interconnections called multilayer PWB's. Connections from layer to layer are typically made by plated-through holes.
Conductor patterns can be formed using either subtractive or additive processes. In a typical subtractive process, a photoresist layer is applied to the copper foil portion of a copper foil-clad epoxy fiberglass substrate and patterned by exposure to ultraviolet light through a stencil-like film artwork mask. The exposed areas of the photoresist are polymerized. The unexposed, unpolymerized areas are removed by a chemical developing solution, leaving areas of copper having the desired conductor pattern underneath the protective barrier of the remaining polymerized photoresist. The exposed copper is then electroplated or etched away (i.e. "subtracted") and the remaining photoresist removed to expose the conductor pattern.
An additive process for forming the conductor pattern starts with an insulating substrate, typically a plastic laminate, throughout which is dispersed a catalyst capable of initiating metal plating on the substrate; typical catalysts are palladium-based materials. The catalytic substrate, referred to as a "fully additive base material," is coated with photoresist and the photoresist is patterned as described above. The holes through the resist formed when the unpolymerized resist is washed away are then filled with metal using electroless plating techniques. Since the conductors are produced by the addition of metal, rather than subtraction as in etching, the process is called "additive".
To promote adhesion between the plated metal and the substrate, the substrate typically is coated with a catalytic adhesive prior to application of the photoresist. The adhesive is usually a resin blend containing the plating catalyst and a colloidal suspension of rubber. The adhesive is treated with a strong etchant which primarily attacks the rubber component, etching it almost entirely. This treatment creates catalytic micropores in the adhesive that promote adhesion between the plated metal and the substrate surface.
In addition to the fully additive base material described above, semi-additive base materials, which are also adhesive-coated, can be used as the substrate material. Both the semi-additive material and its adhesive lack the dispersed catalyst. They are rendered microporous as described above and then immersed in a solution containing a precursor for the catalyst. The precursor is subsequently activated to expose catalytic sites for the plating operation.